I use a CableX box mounted just below the fuse box and a short cable they made up for me. The setup works good but it is still a mechanical speedo.
But what I really use for driving is a scan gage, velcroed just under the speedo. You can set it dead accurate with a GPS and know exactly how fast you are going. (It also displays any 4 quantities that are on the serial bus, I usually run speed, RPM, Fuel level, and water temp.)
http://www.scangauge.com/products/
I also has all the trip functions, MPG functions, you can even make up new gages.
TR